Separation of the constituents of air



Sept. 20, 1949. c. c. vAN'NUYs SEPARATION OF CONSTITUENTS OF AIR Filed Jan. 8. 1944 www NR. AW QN Y Ew A WW No E INVENTR. (/aua La. Ma /I -f BY l l A TTOR Nh' "f5 Patented Sept. 20, 1949 ,SEPARATION OF THE CONSTITUENTS F AIR Claude C. Van Nuys, Greenwich, Conn., assigner to Air Reduction Company, Incorporated, New York, N. Y., a corporation of New York Application January 8, 1944, Serial No. 517,536

Claims. (Cl. (i2-175.5)

This invention relates to the separation of atmospheric air into its constituents by liquefaction and rectification, and particularly to improvements in the method and apparatus therefor.

In commercial liquefaction methods of separating oxygen from the atmosphere as heretofore practised, the necessary refrigeration to attain low temperatures is obtained by compression and subsequent expansion of the air treated. This necessitates initial compression of the air to relatively high pressure of the order of -20 atmospheres. Such compression can be attained only in reciprocating compressors which require oil lubrication. The oxygen of the airunder such pressures and at the temperatures resulting from compression causes decomposition of the lubricating oil and the introduction to the air stream of substantial proportions of hydrocarbons in addition to any which maybe present. The use of special lubricating oils does not avoid this source of contamination.

Since the hydrocarbons are not removed from the air stream by the usual treatment intended to separate moisture and carbon dioxide, they tend to accumulate and eventually nd their way to the compartment of the apparatus where liquid oxygen accumulates. The presence of such material in contact with pure oxygen, whether liquid or gaseous, is undesirable. Hence very elaborate attempts have been made heretofore to eliminate the hydrocarbons at some point intermediate compression ofthe air and the formation of liquid or vapor rich in oxygen in which the hydrocarbons might accumulate. Such attempts involve operations which interfere with the eiliciency of the cycle and the use of necessarily complicated equipment. Furthermore, these attempts have failed substantially to achieve the desired object. It is still possible for hydrocarbons derived from the lubricating oil t0 accumulate at points in the apparatus where gases or liquids rich in oxygen are present.

It is the object of the present invention to provide a simple and eiective method and apparatus whereby the'presence of hydrocarbons resulting from decomposition of oil in the compression of the air can be eliminated and further improvements in the separation of oxygen from the atmosphere can be obtained.

Another object of the invention is the provision of a method and apparatus whereby the power required to maintain refrigeration necessary for the liquefaction oi air and the separation of the constituents thereof is materially reduced, thus affording a more economical procedure for the recovery of oxygen and nitrogen from the atmosphere.

Other objects and advantages of the invention will be apparent as it is better understood by a reference to the following specication and the accompanying drawing, which illustrates diagrammatically an apparatus suitable for the practice of the invention.

In accordance with the present invention, the air to be separated is not utilized to produce sufcient refrigeration to maintain the cycle. I employ instead a separate nitrogen cycle which acts as a heat transfer agent everywhere in the cycle. The .air need be compressed in accordance with my invention only to pressures not substantially higher than three atmospheres absolute. Usually a pressure of two atmospheres, or suiiicient pressure only to ensure movement of the gas through the apparatus is needed to accomplish the purpose of the invention. Such pressures are easily attainable in various types of blowers which require no internal lubrication. Hence no lubricating oil and decomposition products thereof are mingled with the air which is eventually separated to recover oxygen therefrom. The procedure as hereinafter described eieetively increases the proportion of oxygen which can be recovered from the atmosphere and the purity of the` product.

The nitrogen which aiords the refrigerative effect is compressed, circulated and expanded in such a way as to provide suilicient cold so that the air can be liquefied at low pressure. The nitrogen of the refrigeration cycle does not mingle at any point with the air undergoing separation. Hence the presence of hydrocarbons in the nitrogen cycle is not inimical to the m'ethod and aiords no possibility of accumulation of hydrocarbons in the presence of oxygen or products rich in oxygen.

However, in accordance with the present invention, hydrocarbons may be eliminated also from the nitrogen cycle, since it is unnecessary to compress the nitrogen to pressures higher than those attainable in available types of blowers.

-The necessary refrigerative eiiect can be provided readily by compression of the nitrogen to relatively low pressures, generally not exceeding 5 atmospheres absolute, and the subsequent expansion of a portion of the nitrogen with external Work in a suitable engine or turbine. Higher pressures may be used, but it is usually inadvisable for economic reasons to compress the nitrogen to pressures higher than 8 atmospheres absolute, since the additional work expended aords no advantages in the system. While a reciprocating compressor can be employed to compress the nitrogen to the relatively low pressures necessary, a suitable blower is ordinarily satisfactory for the purpose.

That considerable economy is possible in the operation of the procedure as described is readily evident. In commercial systems for the liquefaction and separation of the constituents of air such as the well known Claude system in which a part of the air is expanded with external work, it is necessary usually to compress the air initially to a pressure approximating 20 atmospheres absolute. In the engine, a portion of the air is expanded to approximately atmospheres absolute, or in a. ratio of 4:1. The refrigerative effect obtained is a function of the ratio of expansion. Hence initial compression of the gas which is employed to attain refrigerative eiect to a pressure of 4 atmospheres absolute and expansion of a portion thereof to a pressure of latmosphere absolute will afford the same amount of refrigerative effect in respect to a given volume of the gas. It is not possible to take advantage of this condition where the gas to be separated, such as air, is utilized also in maintaining refrigeration. The advantage can be secured, however, by utilizing nitrogen in a separate refrigeration cycle in accordance with the invention as herein described.

The procedure will be readily understood by reference to the drawing, in which 5 indicates a rectication column having the usual trays 6 and bubble caps 1. The column is provided at its top with an outlet 8 through which the eiiluent rich in nitrogen escapes. Liquid oxygen accumulates` in the bottom of the column, and the vapors rise therefrom through the trays 6 and bubble caps 1 in contact with liquid flowing downwardly over the trays.

To provide this liquid, air, previously compressed to the initial pressure, preferably not greater than 3 atmospheres absolute in any suitable apparatus such as a blower' (not shown) is introduced through a pipe 9 to an exchanger I0 and travels about pipes II and I2 therein in heat exchange relation with cold products of the separation. The air thus cooled is delivered through a pipe I3 to the bottom of a backward return condenser I4 having tubes I 5 and baffles I6 therein. The air flows upwardly through the tubes, being subjected therein to backward return condensation by heat exchange with nitrogen liquid and vapors supplied through a pipe I1. The liquid from the condenser I4, which isenriched in oxygen, is delivered by a pipe I8 controlled by a valve I9 to an intermediate level of the column 5.

The gaseous residue of the` air from the tubes I5, consisting principally of nitrogen, is delivered by a pipe 20 to a condenser 2| containing tubes 22 through which the residue flows in indirect heat exchange relation with liquid nitrogen supplied through a pipe 23. The gaseous residue is condensed by heat exchange with the liquid nitrogen and is delivered by a pipe 24 controlled by a valve 25 to the top of the column 5 and affords the reux nitrogen liquid required to effectively separate substantially all of the oxygen from the air treated. The surplus liquid nitrogen from the condenser 2| overows through the pipe` I1, together with the pure nitrogen vapor formed in condenser 2I.

The eilluent nitrogen from the column is delivered by the pipe 8 to the tubes II of the ex- 4 changer IIT and is withdrawn through a pipe 26 to any suitable receptacle or to the atmphere. The oxygen product may be withdrawn as a liquid through a pipe 21 controlled by a valve 28 and delivered by a liquid pump 28 to a pipe 30 which delivers it to the tubes I2 of the exchanger I0 wherein the liquid is vaporized. The oxygen product is withdrawn through a pipe 3| and delivered to any suitable storage receptacle. Alternatively, oxygen in the form of vapor may be Withdrawn through a pipe 32 controlled by a valve 33 and delivered to the pipe 30. Liquid oxygen may be withdrawn through a pipe 34 controlled pressed nitrogen is delivered by a pipe 31 to a cooler 38. Thence the nitrogen is delivered by a pipe 39 to an exchanger 40 and travels about tubes 4| therein where it is further cooled by heat vexchange with cold nitrogen vapor returning in the cycle. The cold nitrogen leaves the exchanger by a pipe 42 and a portion thereof is delivered to a coil 43 which is immersed in the liquid oxygen accumulating in the bottom of the column 5. The resulting heat vexchange cools the nitrogen and causes vaporization of a. portion of the liquid to maintain the vapor which rises through the column. The partially liquefied nitrogen is then delivered through pipes 44 and 45 to the tubes 46 of a liqueer 41 where it is liqueed lby heat exchange with cold nitrogen delivered to the liqueer as hereinafter described. The liquid nitrogen from the liqueer 41 is delivered by a pipe 48 through a throttle valve 49 and thence through the pipe 23 to the condenser 2| aiording the liquid nitrogen necessary to liquefy the uncondensed vapor discharged from condenser i4 through pipe 20 for use as nitrogen reux for the column 5 and also to effect the initial backward return condensation of the air entering the system. The nitrogen vapor from the condensers I4 and 2| escapes through a pipe 5| to the liquefler 41 affording a part of the necessary refrigeration to complete the liquefaction of the nitrogen in the tubes 46.

A portion of the nitrogen fr'om the pipe 42 is delivered by a pipe 52 controlled by a valve 53 to a pipe 54 which supplies an expansion engine or turbine 55. In the expansion engine or turbine, the nitrogen is expanded from its initial pressure, for example 4 atmospheres absolute, to a pressure of approximately 1 atmosphere absolute with external work, with the result that the nitrogen is reduced to a relatively low temperature, thus affording the refrigeration which is necessary for the maintenance of the cycle. The exhaust from the expansion engine or turbine is delivered through a pipe 56 controlled by a valve 51 tothe liqueer 41 which is also supplied, as hereinbefore described, with cold nitrogen vapor returning through the pipe 5I. The cold nitrogen thus utilized in the liquefaction of nitrogen is delivered through a pipe 58 to the exchanger 4|! and after giving up its cold -by circulating through the tubes 4|, returns through a pipe 59 to the compressor 36 for further use in the cycle.

-A portion of the nitrogen in the exhaust from the expansion engine or turbine 55 may be liqueiled. Such liquid is withdrawn through a pipe 60 controlled by a valve 6| and is delivered to the pipe 48 where it joins liquid from the liquefler 41 after the latter has passed the throttle valve 49. Hence the liquid portion of the exhaust is utilized in a condenser 2| at the top of the column. Pipe 60 and valve 6I may also be utilized for sending a portion of the turbine or engine exhaust in the vapor form directly to pipe 23 and condenser 2l for regulation of the distribution of refrigerative effect.

The cold nitrogen from the coil 43 which normally passes through the pipe 45 to the liquefier 41 is still at the initial pressure and its temperature is somewhat lower than the temperature of the nitrogen diverted through the pipe 52. In certain phases of the operation, it may be desirable to divert some of this cold nitrogen to the expansion engine or turbine 55. Consequently a pipe 62 controlled by a valve 63 connects the pipe 44 with the pipe 54. Any desired portion of the cold high pressure nitrogen may thus be directed to the expansion engine or turbine 55. If desired, all of the nitrogen at the initial pressure may be delivered through the coil 43 and the desired portion thereof may then be diverted to the expansion engine or turbine 55 to effect the desired reduction of pressure with external work and thus aiord the refrigeration required for the operation of the system.

The nitrogen in the refrigeration cycle does not at any point mingle with the air undergoing separation or the products thereof. Nitrogen separated from the air is withdrawn, as is also the oxygen product, independently of the refrigeration cycle. Since the nitrogen employed for refrigeration travels in an independent cycle, it is immaterial whether or not it may carry hydrocarbons. These hydrocarbons, if present, cannot mingle with the air undergoing separation and thus arrive at a point in the system where oxygen or products rich in oxygen are present. Thus the liquid oxygen which accumulates in the bottom of the column 5 is free from any contaminating hydrocarbons which might have been introduced owing to the necessity of empliying oil in the initial compression of the air. The use of the blower to compress the air obviates that possibility.

A further major advantage of the method and apparatus as described is, as already indicated, that the initial pressure of the nirtogen may be relatively low, as compared with pressures normally employed in commercial systems. An initial pressure of 4 atmospheres absolute requires much less power input to maintain the cycle than a pressure of 20 atmospheres absolute. Even pressures as high as 8 atmospheres absolute in the initial compression of the nitrogen aiord material savings. Nevertheless it is possible to supply all of the required refrigeration by expjansion of a portion of the nitrogen from its initial pressure to a pressure of approximately 1 atmosphere absolute. Since the nitrogen employed in maintaining refrigeration does not in any manner enter the rectification or affect it, it is evident that the cycle can be maintained and the desiredseparation eiected on a, very economical basis.

The invention as described affords a practical and simple solution of a problem which has existed for many years in the commercial production of oxygen by liquefaction and rectification. It permits substantially total recovery of the oxygen content of the air, since the nitrogen reilux which is supplied at the top of the column prevents the escape of oxygen in the eiuent. Finally it affords an economical procedure for the recovery of oxygen and nitrogen from the atmosphere.

Various changes may be made in the details of procedure and in the apparatus as described without deporting from the invention or sacricing the advantages thereof.

I claim:

1. The method of separating the constituents of atmospheric air by liquefaction and rectification which comprises maintaining a closed nitrogen cycle, compressing air without contaminating the same with lubricant, subjecting the air to liquecation by indirect heat exchange with liquid nitrogen of the nitrogen cycle and subsequently to rectification to separate liquid oxygen and a gaseous eiuent, compressing nitrogen of the closed nitrogen cycle to a relatively low pressure, expanding a portion of the compressed nitrogen to a lower pressure with external Work, liquefying the other portion of the compressed nitrogen by indirect heat exchange with the expanded portion and thereby supplying the liquid nitrogen utilized in the liquefaction of the air.

2. The method of separating the constituents of atmospheric air by liquefaction and rectification which comprises maintaining a closed nitrogen cycle, compressing air Without contaminating the same with lubricant, subjecting the air to liquefaction by indirect heat exchange with liquid nitrogen of the nitrogen cycle and subsequently to rectification to separate liquid oxygen and a gaseous effluent, compressing nitrogen of the panding a portion of the compressed nitrogen to a lower pressure with external work, liquefying the ,other portion of the compressed nitrogen by indirect heat exchange with the expanded portion and with cold nitrogen vapor from preceding liqueed portions of the nitrogen, and thereby supplying the liquid nitrogen utilized in the liquefaction of the air.

3. The method of separating the constituents of atmospheric air by liquefaction and rectification which comprises maintaining a closed nitrogen cycle, compressing air without contaminating the same with lubricant, subjecting the air to liquefaction by indirect heat exchange'with liquid nitrogen of the nitrogen cycle and subsequently to rectication to separate liquid oxygen and a gaseous eiiluent, compressing nitrogen of the closed' nitrogen cycle to a pressure not materially exceeding 5 atomospheres absolute, expanding a portion of the compressed nitrogen to a lower pressure with external work, liquefying the other portion of the compressed nitrogen by indirect heat exchange with the expanded portion and thereby supplying the liquid nitrogen utilized in the liquefaction of the air.

4. The method of separating the constituents of atmospheric air by liquefaction and rectification which comprises maintaining a closed nitrogen cycle, compressing air without contaminating the same with lubricant, subjecting the air to liquefaction by indirect heat exchange with liquid nitrogen of the nitrogen cycle and subsequently to rectication to separate liquid oxygen and a gaseous efliuent, compressing nitrogen of the closed nitrogen cycle to a pressure not materially exceeding 8 atmospheres absolute, expanding a portion of the compressed nitrogen to a lower pressure with external Work, liquefying the other portion of the compressed nitrogen by indirect heat exchange with the expanded portion and thereby supplying the liquid nitrogen utilized in the liquefaction of the air.

of atmospheric air by liquefaction and rectifica- A tion which comprises maintaining a closed nitrogen cycle, compressing air without contaminating the same with lubricant, subjecting the air `to liquefaction by indirect heat exchange with liquid nitrogen of the nitrogen cycle and subsequently with cold nitrogen vapor from preceding liquefiedportions of the nitrogen of the closed nitrogen cycle, and thereby supplying the liquid nitrogen utilized in the liquefaction of the air.

6. The method of separating the constituents of atmospheric air by liquefaction and rectification which comprises maintaining a closed nitrogen cycle, compressing air without contaminating the same with lubricant, subjecting the air to liquefacticn by indirect heat exchange With liquid nitrogen of the nitrogen cycle and subsequently to rectiiication' to separate liquid oxygen and a gaseous effluent, compressing nitrogen of the closed nitrogen cycle to a pressure not materially exceeding 8 atmospheres absolute, expanding a y portion of the compressed nitrogen to a lower pressure with external work, liquefying the other portion .of the compressed nitrogen by indirect heat exchange with the expanded portion and with cold nitrogen vapor from preceding liquefied portions of the nitrogen of the closed nitrogen cycle, and thereby supplying the liquid nitrogen utilized in the liquefaction of the air.

'7. The method .of separating the constituents of atmospheric air by liquefaction and rectification which comprises maintaining a closed nitrogen cycle, compressing nitrogen of the closed nitrogen cycle to a relatively low pressure, expanding a portion of the compressed nitrogen to a lower pressure with external work, liquefying the other portion of the compressed nitrogen by indirect heat exchange with the expaded portion thereby maintaining a constant supply of liquid nitrogen to afford refrigeration, compressingair, vaporizing the liquid nitrogen of the closed nitrogen cycle by indirect heat exchange of the compressed air therewith ,obtaining thereby liquid fractions and rectifying the liquid fractions obtained containing the oxygen and nitrogen constituents .of the air to separate such constituents.

8. The method of separating the constituents of atmospheric air by liquefaction and rectiiication which comprises maintaining a closed nitrogen cycle, compressing nitrogen of the closed nitrogen cycle to a relatively low pressure, expanding a portion of the compressed nitrogen to a lower pressure with external work, liquefying the other portion of the compressed nitrogen by indirect heat exchange with the expanded portion thereby maintaining a constant supply of liquid nitrogen to afford refrigeration, compressing air, vaporizing the liquid nitrogen of the closed nitrogen cycle by indirect heat exchange of the compressed air therewith obtaining thereby liquid fractions, mixing the nitrogen vapor resulting from such heat exchange with the expanded nitrogen and rectifying the liquid fractions obtained containing the oxygen and nitrogen constituents of the air to separate such constituents.

9. The method of separating the constituents of atmospheric air by liquefaction and rectication which comprises maintaining a closed nitrogen cycle, compressing nitrogen of the closed nitrogen cycle to a pressure not materially ex- -ing the oxygen and nitrogen constituents of the air to separate such constituents.

10. The method of separating the constituents of atmospheric air by liquefaction and rectiilcation which comprises maintaining a closed nitrogen cycle, compressing nitrogen of the closed nitrogen cycle to a pressure not materially exceeding 8 atmospheres absolute, expanding a portion of the compressed nitrogen to a lower pressure with external work, liquefying the other portion of the compressed nitrogen by indirect heat exchange with they expanded portion thereby maintaining a constant supply of liquid nitrogen to aord refrigeration, compressing air, Vaporizing the liquid nitrogen of the closed nitrogen cycle by indirect heat exchange of the compressed air therewith obtaining thereby liquid fractions and rectifying the liquid fractions obtained containing the oxygen and nitrogen constituents of the air to separate such constituents.

' CLAUDE C. VAN NUYS.

REFERENCES CITED The following references are of record in th le of this patent:

` UNITED STATES PATENTS Van Nuys Oct. 15, 1946 Certicate of Correction Patent No. 2,482,303 September 20, 1949 CLAUDE C. VAN NUYS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 5, line 44, for emphying read employing; line 49, for nirtogen read nitrogen; line 58, for expjansion read expansion; column 6, line 4, for deporting read departing; line 31, after the Words of the insert nitrogen cycle to a relatively low pressure, e; column 7, line 46, for expaded read epancled;

and that the said Letters Patent should be read With these corrections therein that the same may conform to the record of the case in the Patent Oce.

Signed sind sealed this 17 th day of January, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

